There are numerous circumstances where it is desired to protect steel from corrosion by acidic material to which it is exposed. These include the protection of steel exposed to an aqueous acidic liquid such as when steel tubing is used to convey a flow of aqueous liquid.
One industry which has a need for protection of steel against corrosion is oil and gas exploration and production. Steel tubulars used in a borehole may be exposed to an acidic liquid mixture and so also may steel pipelines used to carry produced oil and gas.
The technique of matrix acidising in which the producing formation is treated with acid to stimulate production involves deliberate exposure of borehole steel to acid. This operation may be performed with coiled tubing which is run into a borehole and then used to convey acid down the borehole to the formation. When the operation comes to an end, the steel casing in the borehole and the exterior of the coiled tubing can be exposed to so-called unspent acid flowing back with formation fluids towards the surface.
A conventional approach to the protection of steel against corrosion by an acidic liquid is to contact the steel with a corrosion inhibitor. For example, when conveying an acidic liquid through steel tubing, it is conventional to add a corrosion inhibitor to the flow of liquid as mentioned in many documents including for example U.S. Pat. Nos. 3,773,465, 4,498,997 and 5,120,471.
Organic inhibitors adsorb on the metal surface. Adsorbed inhibitor(s) may influence the rate of corrosion by one or more of several mechanisms: (i) by forming a physical barrier film which restricts the diffusion of species to/from the metal surface, (ii) by blocking anodic and/or cathodic reaction sites directly, (iii) by interacting with corrosion reaction intermediates adsorbed on the surface and (iv) by influencing the electrical double layer that forms at the metal/solution interface.
Adsorption may be physi-sorption which is the result of electrostatic attractive forces between inhibiting organic ions or dipoles and the electrically charged surface of the metal. The surface charge of the metal is due to the electric field at the outer Helmholtz plane of the electrical double layer existing at the metal/solution interface.
Another possibility is that adsorption is by chemi-sorption, which takes place more slowly than electrostatic adsorption and with a higher activation energy. Chemi-sorption involves electron transfer from electron-rich sites within the structure of the inhibitor molecule(s) to vacant low energy orbitals in the metal. Typically, such electron-rich sites within an inhibitor molecule are heteroatoms with lone pair(s) of electrons or are multiple bonds and aromatic rings so that covalent bonds have electrons in π-orbitals. Because activation energy is required, to bring about chemi-sorption, the extent of chemi-sorption and therefore the efficacy of corrosion inhibition may increase with temperature. It is known that chemi-sorbed acetylenic compounds can react to form polymeric inhibitor films. Such reaction/polymerisation is surface-catalysed.
As illustrated by the US patents mentioned above, corrosion inhibitors are frequently marketed as a mixture containing materials which inhibit corrosion and other materials which enhance inhibition, even though these other materials do not function as corrosion inhibitors (or are less efficacious) if used alone. In some cases these mixtures are proprietary and their exact composition is not made public.
A mixture which contains a chemi-sorbing corrosion inhibitor may also include non-ionic or cationic surfactants to assist deposition on steel, quaternary nitrogen compounds, amines (which will protonate to quaternary nitrogen under acidic conditions) and organic solvent. Some oilfield corrosion inhibitor products considered to provide good inhibition performance at high temperatures are mixtures which make use of a synergistic combination of an amine and an acetylenic alcohol.
Materials used in a corrosion inhibiting mixture may include materials which are considered to damage an oil reservoir by depositing on the reservoir formation. In particular surfactants in a corrosion inhibiting mixture may assist deposition of corrosion inhibitor on steel, yet be regarded as potentially reducing production from a reservoir. Consequently there is a conflict between a desire to protect steel tubing by means of inhibitor compositions and a desire to protect the reservoir from exposure to such inhibitor compositions.
The corrosion inhibiting effect of an inhibitor or corrosion inhibiting mixture can be tested in various ways. One direct method of testing is to use a test piece which is a sample of the steel to be protected, customarily referred to as a “coupon”. This coupon is exposed for a measured length of time to an acidic solution containing a known concentration of corrosion inhibitor. The loss in weight of the coupon is measured and expressed as weight loss per unit surface area. The coupon is also examined for localised pitting and the extent of pitting may be expressed as a numerical value: the so-called pitting index.
There are a number of other ways to measure corrosion by an acidic solution. These include linear polarization resistance measurement which was first proposed by M Stern and A L Geary in “Electrochemical Polarization: I. A Theoretical Analysis of the Shape of Polarization Curves” in J. Electrochem. Soc. Vol 104 pp 56-63 (1957) and followed by Stern: “A Method For Determining Corrosion Rates From Linear Polarization Data” in Corrosion, Vol. 14, No. 9, 1958, pages 440t-444t. In such tests a piece of the steel is used as an electrode and this electrode may be kept moving as a rotating disc, cylinder or cage to simulate flow of the corrosive solution over the steel.
When steel is going to be exposed to a flow of an acidic composition, it is normal practice to test coupons of the steel with various concentrations of corrosion inhibitor in samples of the acidic composition. A concentration of inhibitor which produces an acceptably low weight loss and pitting index is identified and this concentration of inhibitor is then maintained in the flow of acidic composition to which the steel is exposed.